SIC2 - A MOLECULAR PINWHEEL

We present the results of a combined experimental and theoretical stud y of the large-amplitude motion in SiC2 in which the C-2 fragment unde rgoes hindered internal rotation. Stimulated emission pumping (SEP) is used to obtain rovibrational term energies for levels with up to 14 q uanta of excitation in the large-amplitude vibration. We analyze the S EP data, as well as other available experimental data, using a semirig id bender model that allows for complete internal rotation within a tr iatomic molecule. From the least-squares fitting of this model to the data, we determine the potential energy along the minimum energy path of the large-amplitude vibration, the harmonic energies of the small-a mplitude coordinate. The fitting is aided by results obtained from ab initio calculations we perform on the triangular and linear configurat ions of the molecule. The current data set is consistent with a large- amplitude potential energy function in which the energy difference bet ween the triangular and linear configurations is 1883 cm(-1). The stat istical error on this energy difference is 22 cm(-1), but we estimate the physical uncertainty to be about 200 cm(-1). This result is in exc ellent agreement with the energy difference of 1819 cm(-1) we obtain i n our best ab initio calculations. The semirigid bender fitting and ou r best ab initio calculations are also both consistent with a potentia l energy function having no local minimum at linearity.